Abstract

We have previously demonstrated a role for the reorganization of the actin cytoskeleton in store-operated calcium entry (SOCE) in human platelets and interpreted this as evidence for a de novo conformational coupling step in SOCE activation involving the type II IP 3 receptor and the platelet hTRPC1-containing store-operated channel (SOC). Here, we present evidence challenging this model. The actin polymerization inhibitors cytochalasin D or latrunculin A significantly reduced Ca 2+ but not Mn 2+ or Na + entry into thapsigargin (TG)-treated platelets. Jasplakinolide, which induces actin polymerization, also inhibited Ca 2+ but not Mn 2+ or Na + entry. However, an anti-hTRPC1 antibody inhibited TG-evoked entry of all three cations, indicating that they all permeate an hTRPC1-containing store-operated channel (SOC). These results indicate that the reorganization of the actin cytoskeleton is not involved in SOC activation. The inhibitors of the Na +/Ca 2+ exchanger (NCX), KB-R7943 or SN-6, caused a dose-dependent inhibition of Ca 2+ but not Mn 2+ or Na + entry into TG-treated platelets. The effects of the NCX inhibitors were not additive with those of actin polymerization inhibitors, suggesting a common point of action. These results indicate a role for two Ca 2+ permeable pathways activated following Ca 2+ store depletion in human platelets: A Ca 2+-permeable, hTRPC1-containing SOC and reverse Na +/Ca 2+ exchange, which is activated following Na + entry through the SOC and requires a functional actin cytoskeleton.